Computing devices, such as notebook computers, personal data assistants (PDAs), kiosks, and mobile handsets, have user interface devices, which are also known as human interface devices (HID). These interface devices may include touch-sensor devices such as a touchpad or a touch screen. Conventional touchpads or touch screens allow a user to interact using his or her finger. However, a stylus may be useful for interacting with a touch-sensor device. A stylus provides two distinct advantages to a user compared to interacting with a touch-sensor device using only a finger. First, a stylus has a smaller area of contact with the touch-sensor device than a finger. A typical finger may have an area of contact of approximately 10 millimeters in diameter, where a stylus may have an area of contact of approximately 1 square millimeter (mm2). The smaller area of contact offered by the stylus allows the user to more accurately interact with the desired portion of the touch-sensor device. The second advantage is that the user can better see where he or she is interacting with the touch-sensor device because the stylus covers less of the display screen. A user's finger and hand may obscure a portion of the touch-sensor device when the user reaches out to contact the touch-sensor device. A stylus, especially near the point of contact with the touch sensor, is ideally much thinner than a finger and will not obscure as large a portion of the touch-sensor device while being used.
Several different stylus alternatives are currently available, but each has drawbacks which make them less than ideal. The first is a stylus designed for use with a resistive touch screen. A resistive touch screen generally is made from two layers of transparent conductive material (e.g., Indium Tin Oxide, stannous oxide, carbon nanotubes, graphene, PEDOT-PSS) separated by air. A typical stylus may include a piece of plastic with a radiused end made from a slippery polymer (e.g., high-density polyethylene (HDPE), ultra high molecular weight polyethylene (UHMWPE), TEFLON™, RULON™) to prevent scratching of the surface of the screen. Nevertheless, with a resistive touch screen anything hard enough to cause the top layer to deflect into contact with the bottom layer can be used as a stylus, such as a pencil, pen, fingernail, credit card, etc. The problem in this case is not with the stylus itself, but with the resistive touch screen. Resistive touch screens are failure prone due to wear out or fracture of the indium tin oxide (ITO) material from repeated flexing. The smaller radius of the stylus (relative to that of a finger) decreases the bend radius of the top layer of such a touch screen, accelerating the failure of the touch screen from repeated flexing of the ceramic ITO layer. Resistive touch screens also suffer high light loss due to the air gap present between the active layers.
The second type of stylus is an inductively-powered stylus using technology from a company such as Wacom Co., Ltd. of Japan or Sensopad Limited of the United Kingdom. In these systems, there may not be a touch sensitive screen in place, though the inductive stylus does not preclude its use or presence. The inductively-powered stylus includes an inductor-capacitor (LC) tank circuit. Magnetic coils, either below or around the display surface, are energized and couple power to the stylus. When the power to these coils in the display is turned off, the energy present in the tank circuit is then output by the stylus. As the magnetic field from the stylus collapses, the stylus couples power back into the coils in the display where the location can be sensed. The inductively-powered stylus requires additional electronic circuitry within the stylus itself, thereby increasing the cost of the stylus. Additionally, the separate coils in the display that couple power to the stylus are not very efficient at power transfer. This limits the operable life in portable applications due to battery drain. The coils also increase the bulk and thickness of the display section.
Another type of stylus is a tethered stylus similar to that from Ingenico of France as used with their POS (point-of-sale) systems. The stylus may be tethered to the POS system primarily to prevent it from being removed, but it also requires an electrical connection through the tether to provide power and allow proper communication with the host system. Here the touch screen is presented with a sinusoidal signal at around 250 kHz that is driven alternately horizontally and vertically across the ITO touch screen surface. Since this is driven as a current through a non-inductive media, the touch screen forms a linear voltage divider (similar to that of a resistive touch screen). The relative field amplitude at the surface of the touch screen is directly related to the relative voltage amplitude remaining in each of the X and Y positions. The stylus for this consists of a mechanical switch at the contact end of the stylus to indicate the presence of a stylus on the touch screen, and an antenna plus amplifier contained within the stylus body to receive a signal from the panel. The amplitude of this signal is communicated back to the host system where it is then analyzed in both the X and Y directions to determine a touch location for the stylus.
Another type of tethered stylus is offered by 3M of Minnesota or Cirque Corporation of Utah. Touch International of Texas is one company that uses the Cirque solution. Here the stylus is powered through the tether and is used to inject a signal into the touch screen. The touch screen then uses the same sensing techniques used to detect a finger to also detect the stylus. In the case of the 3M solution, the stylus is configured to couple a dynamic current opposite that presented by a human body. This allows the controller to be able to distinguish the difference between the two. While this design may be functional, additional power is required to drive the circuitry in the stylus and the tether itself may not be desirable in many applications.
Another stylus type is a merging of the inductive powering mechanisms, with that of the tethered stylus technique. Here, instead of using a tether or cable to provide power to the pen, the power is delivered through inductive coupling. Once the pen has power, the power is used to create an electrostatic field that can be sensed by the touch screen. This design suffers from similar drawbacks to those discussed above, such as increased cost and stylus complexity.
Yet another type of stylus is a completely passive conductive stylus as illustrated in FIG. 1. Such a conductive stylus can couple the natural capacitance of the user's body to the touch-sensor device. The stylus includes a gimbal-jointed tip to allow free movement, but still requires a large contact area in order to effectively couple the body's capacitance. The large contact area effectively eliminates many of the benefits of using a stylus, such as accurate interaction and increased visibility. FIG. 1 illustrates a completely passive conductive stylus.